Digital voice communications may be implemented in a packet-switched network. Methods associated with two such implementations may include those defined in voice over Internet protocol (VoIP) specifications H.323 and Internet Engineering Task Force (IETF) Request for Comments (RFC) 2543. Additional information regarding H.323 may be found in International Telecommunication Union (ITU) Standard H.323—Version 5 “Packet-based Multimedia Communications Systems” (July 2003). Additional information regarding a session initiation protocol (SIP) may be found in IETF RFC 2543, “SIP: Session Initiation Protocol” (1999).
H.323 and session initiation protocol definitions may include voice signaling methods such as initiating, modifying, and terminating a voice call. Signaling traffic may be present on a link whether or not active voice traffic is present on the link, and can be sensitive to delay. While voice packets may be sent using a real-time protocol, perhaps implemented with a user datagram protocol, signaling packets are usually sent through a transport control protocol.
These factors may be particularly significant as they relate to wireless packet transmission methods such as those incorporated in an Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and in an IEEE 802.11E standard. Additional information regarding the IEEE 802.11 standard may be found in “ANSI/IEEE Std. 802.11, Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” (published 1999; reaffirmed June 2003). Additional information regarding the IEEE 802.11E standard may be found in “IEEE 802.11E Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements (published 2005).
Voice and signaling packets may be prioritized differently according to quality of service (QoS) methods defined in the 802.11E standard. Voice packets may be sent with an access category-voice (AC-VO) channel access category that is prioritized relatively higher than an access category-video (AC-VI) channel access category used for voice signaling.
Such differential prioritization of voice and signaling packets may be considered together with a state of reduced power consumption referred to as a power-save mode of operation. The power-save mode of operation may be utilized by a battery-powered wireless device to extend an operating time between battery recharge cycles.
A particular type of power-save mode of operation referred to as an automatic power-save delivery (APSD) mode is described in the IEEE 802.11E standard. APSD is a mechanism for delivery of downlink frames from a wireless access point operating in a QoS mode (a “QAP”) to a station (STA) operating in both the QoS mode and in the power-save mode (a “QSTA”). Although two APSD sub-modes are described, scheduled and unscheduled, the unscheduled or “triggered” (U-APSD) mode may predominate in commercial applications.
A QSTA operating in the U-APSD mode may awaken periodically for a service period of time. A service period is a contiguous period of time during which a set of one or more downlink frames is sent to one or more QSTAs.
The IEEE 802.11E specification defines two attributes associated with access category traffic, trigger-enabled access category and delivery-enabled access category. Trigger-enabled access category refers to an attribute associated with traffic in the uplink. A packet in the uplink associated with a trigger-enabled access category may trigger an unscheduled service period if one is not already in progress. When an access category associated with a QAP is delivery-enabled, the QAP may be allowed to deliver traffic associated with the access category to a QSTA in an unscheduled service period triggered by the QSTA.
An unscheduled service period may begin when the QAP receives a QoS-data packet (e.g., a prioritized voice packet or a prioritized voice signaling packet) or a QoS-null packet associated with an active access category in the uplink. An access category may be thought of as a level of priority associated with a particular class of traffic. The unscheduled service period may end when the QSTA receives a QoS data packet or a QoS null packet with an end of service period (EOSP) subfield set to 1 in the downlink. Once the QAP has received an acknowledgment to the latter packet, the QAP ceases transmission of packets to the QSTA using this mechanism until the next service period. Unscheduled APSD thus requires a traffic stream of QoS data or QoS null packets in the uplink to trigger a transmission of traffic in the downlink.
The QAP may buffer delivery enabled traffic until it receives a packet from the QSTA associated with a trigger enabled access category. The QAP may then transmit the buffered packets to the QSTA. Buffered traffic associated with an access category with a U-APSD attribute set to “non-delivered” may continue to be buffered at the QAP and may not be forwarded to the QSTA upon a receipt at the QAP of the packets associated with the trigger-enabled AC. The QAP may notify the QSTA about buffered non-delivered traffic using a deliver traffic information message (TIM) in a beacon packet associated with a standard, non-APSD power-save mode of operation. A TIM may not be set for traffic associated with a delivered traffic access category unless all ACs are delivery enabled, however. The QSTA should thus use upstream traffic to trigger a delivery of traffic from the QAP buffer.
A problem with using VoIP with U-APSD may result from the fact that that the 802.11E standard was designed with voice streams in mind, but does not necessarily define how the QSTA can receive unexpected downstream signaling traffic to initiate a voice stream. Although static access category configurations may be implemented, these may result in large delays to signaling traffic and/or increased power consumption when coupled with APSD use. A user may experience symptoms of a bad call connection attempt (e.g., an “other side not available” or a “line broken” indication).